(1) Field of the Invention
The present invention relates to a color developer for a leuco pigment. More particularly, the present invention relates to a color developer for a leuco pigment, which comprises an oligomer or polymer of a vinyl phenol. Furthermore, the present invention relates to the use of an oligomer or polymer of a vinyl phenol as a color developer for a leuco pigment in a thermosensitive recording material, an electrostatic photographic recording material or the like.
(2) Description of the Prior Art
Compositions comprising a leuco pigment and a color developer composed of an acidic substance have heretofore been used widely for various methods of recording information, for example, thermosensitive recording, pressure-sensitive recording and electrostatic photographic recording. For example, a recording material comprising a recording layer formed on an appropriate substrate by dispersing a leuco pigment which is colorless or has a light color in the normal state and a phenol which is solid at normal temperatures but fusible under heating into a polymeric binder to form independent dispersed phases of the leuco pigment and the phenol, has been used as a recording element for thermosensitive recording.
However, known phenol type color developers for leuco pigments are insufficient and defective in some points. For example, when the total amount coated of the leuco pigment and the phenol type color developer is constant, in order to obtain a recorded image having a certain density, it is necessary to use the expensive leuco pigment in a considerably larger proportion. Accordingly, the cost of a recording material, for example, the above-mentioned thermosensitive recording element, is increased and fogging is readily caused in the thermosensitive recording element.
It is known that among these known phenol type color developers, some phenols, for example, bis-2,2-(4-hydroxyphenyl)propane, have a reduced tendency to cause fogging. However, these phenols have a relatively high melting point, and therefore, they are still unsatisfactory in that a high temperature heat source is necessary for effecting the thermosensitive recording.